Membrane filtration

ABSTRACT

A method for membrane filtration of effluent water containing suspended matter reduces membrane clogging and improves filtration capacity of the membrane. The method includes adding a dose of a preselected coagulation reactant to the effluent before the effluent passes over the membrane. The added dose is a fraction of the coagulating reactant dose (X) that would render the zeta potential of the effluent equal to zero. The range of the added dose is X/30-X/80. The suspended matter in the effluent is subjected to the added dose of coagulation reactant for destabilizing the suspended matter.

FIELD OF THE INVENTION

The present invention relates to improvements to membrane filtration ofeffluents, especially water, containing suspended matter for the purposeof reducing or preventing the membranes from clogging and of improvingthe filtration capacity of the latter.

BACKGROUND OF THE INVENTION

It is known that filtration (micro-, ultra-, nano- or hyperfiltration)membranes are sensitive to clogging by various types of substances:dissolved substances, such as organic materials, substances in thecolloidal state, such as metal hydroxides, or, in general, substances insuspension (suspended matter or SM). Clogging results in a verysubstantial reduction in the filtration capacity of the membrane, thereduction in capacity not always being reversible, the effectiveness ofmembrane cleaning depending considerably on various factors, such as theeffective residence time of the fouling product on the membrane, itsrelative solubility in the cleaning products and the chemical orphysico-chemical interactions between the said fouling product and thesurface of the membrane, the latter factor being eminently variableaccording to the chemical composition of the polymer constituting themembrane.

It is also known (see especially “Mémento Technique de l'Eau [WaterTechnical Memorandum]”, Volume 1, Chapter 3.1 and Chapter 4.1, publishedby DEGREMONT, 1989) that coagulation makes it easier to remove thesuspended and colloidal matter. In particular, a person skilled in theart knows that coagulation by metal salts makes it possible todestabilize the colloids and to precipitate certain organic materialsafter adsorption, for example on metal hydroxides. There are severalapproaches for characterizing this phenomenon:

-   -   by a coagulation-flocculation test in a laboratory beaker with        various doses of metal salt and estimation, for example, of the        settling rates;    -   by measuring the zeta potential (ZP) and especially the        variation in the said ZP as a function of the doses of metal        salt added, until that dose which makes the ZP zero, and which        therefore corresponds to the required level of treatment for        obtaining optimum coagulation, is determined.

These two approaches lead to a definition of a coagulant dose called the“optimum coagulation dose” which, from the experience gained by thoseskilled in the art, is the dose which allows the best clarificationtreatment of the water being treated and which, consequently, willensure the optimum working conditions of the membrane (that is to saythe least fouling conditions).

The drawback of such a treatment involving this optimum coagulation doseis that this dose is relatively high and has repercussions on theoperating cost of the clarification treatment and also on the cost ofinvesting the corresponding equipment.

Moreover, it should be noted that most membrane suppliers and suppliersof nanofiltration and reverse osmosis equipment insist, for fear of lossof the guarantees associated with the membranes, on feeding them onlywith water having a zero, or at the very least a very low, content ofheavy metals such as divalent or trivalent ions, in particular such asferric ions.

Thus, in the literature there are many publications mentioning the useor the injection of one or more metal salts upstream of the membranetreatments. It should be emphasized that these publications mentioneddoses close to that making the ZP zero or, at the very least, highdoses, close to 30% and more of the said optimum dose for making thesaid ZP zero.

BRIEF DESCRIPTION OF THE INVENTION

The present invention has the objective of providing a process making itpossible to minimize, or at the very least reduce, the clogging ofmembranes and to improve their filtration capacity, while making theprocess more economic. To achieve this result, the technical problems tobe solved are the following:

-   -   how to substantially increase the specific production flux        (l.h¹.m⁻² of membrane);    -   how to produce the least sludge (of hydroxides for example)        resulting from the clarification treatment and, above all:    -   how to reduce the area of the membranes to be fitted, in order        to treat the same volume of water.

The Proprietor has observed, in a really surprising manner for oneskilled in the art, that a dose of coagulation reactant very much lessthan the dose which makes the zeta potential of the water to be treatedzero allows the filtration capacity of the membrane to be considerablyimproved.

The subject of this invention is therefore improvements to the membranefiltration, especially micro-, ultra-, nano or hyperfiltration, ofeffluents, especially water, which contains suspended matter, so as toreduce clogging of the membranes and improve the filtration capacitythereof, characterized in that they consist in adding to the effluent tobe filtered, coagulation reactant which destabilizes the colloidalmatter in suspension. The added dose is a fraction of the dose (X)making the zeta potential zero. The fraction of the added dose is in therange X/30–X/80.

According to an advantageous method of implementing the processaccording to the invention, the dose of coagulation reactant is aroundX/40–X/60.

The field of application of the invention is particularly wide. This isbecause it can be used especially for the membrane treatment of watercoming from various sources, such as for example:

-   -   municipal waste water, after a biological treatment and a        separation allowing less than 20 mg/l of SM to be obtained;    -   water not requiring the prior removal of organic matter and        having a total organic carbon (TOC) content of less than 2 ml/g;        and    -   raw surface water, having a low TOC, a high colloidal        concentration and containing less than 200 mg/l of SM.

The process according to the invention gives excellent results when itis applied to membranes of different shapes (capillary, tubular, planarand spiralled membranes) with an internal or external skin, havingvarious configurations (in a casing, without a casing and immersed in abasin). The invention is also suitable for applications involving thespraying of recreational areas, the reutilization of waste water infactories and, more generally, the upstream pretreatment ofreverse-osmosis desalination plants.

Further features and advantages of the present invention will becomeapparent from the description given below with reference to the appendeddrawing and to the illustrative examples given below. In the drawing:

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic view illustrating the injection of coagulatingreactant, for example a metal salt, especially an iron salt, upstream ofa membrane in a casing into the flow; and

FIG. 2 is also a schematic view illustrating the injection of thecoagulating reactant upstream of an immersed membrane without a casing.

DETAILED DESCRIPTION OF THE INVENTION

In both figures, identical or similar elements are denoted by the samereference numbers.

In the method of implementation shown in FIG. 1, the coagulatingreactant is injected at 2 into the water 1 to be treated and the waterto be treated/coagulating reactant mixture is filtered over the membranein the casing 4. The system includes a recirculation loop 5. Thereference number 3 denotes the outflow of the treated water.

In the example of implementation illustrated in FIG. 2, the coagulatingreactant 2 is injected into the water 1 to be treated, the mixture thenbeing filtered over the membrane 6, without a casing, immersed in abasin containing the water to be treated. The treated water 3 isdischarged by means of a pump.

Given below are two numerical examples of implementation so as to makethe technical aspects and advantages afforded by the present inventionclearly apparent.

Example 1: Treatment of Municipal Waste Water

A biological treatment test was carried out on municipal waste watercoming from a fabric-membrane bioreactor, such as the one described inFR-A-2 775 911.

The quality of the water output by the reactor was the following:

Total COD: 40 mg/l Total BOD₅: <10 mg/l SM: 5 mg/l TOC: 6 mg/l.

The dose which makes the zeta potential (ZP) zero was found bylaboratory trial to be 140 mg/l of FeCl₃ (expressed as pure FeCl₃). Theoptimum dose of coagulating reactant, determined by in-beakerflocculation, to reduce the organic matter (measured by UV absorption at254 nm) was 110 mg/l.

The output of treated water from the fabric-membrane reactor was l m³/h.The steady flux through the capillary ultrafiltration membrane was 32l.h⁻¹.m⁻². When a small amount of coagulating reactant was used, byinjecting into the line 3 mg/l of FeCl₃ (expressed as pure FeCl₃), thesteady flux was 100 l.h¹.m⁻².

The backwashing was carried out with 5 mg/l of chlorine for 30 secondsevery 30 minutes and, from time to time, for example once a month, withammoniacal citric acid. The backwashing water was returned to the top ofthe fabric-membrane bioreactor.

In this example, to treat l m³/h without a coagulating reactant, 35l.h⁻¹.m⁻² was obtained and a membrane area of 28.5 m² was thereforerequired. In contrast, by injecting 3 mg/l of FeCl₃, 100 l.h⁻¹.m⁻² wasobtained and the membrane area required was then 10 m². This representsan improvement in flux of 285% and a saving of 18.5 m², i.e. 65%, ofmembrane at least.

Sludge production was measured to be about 8 mg/l with 3 mg/l of FeCl₃instead of 5 mg/l without the addition of a coagulating reactant;however, this remained very much less than the amount which would beproduced with 125 mg/l of FeCl₃, i.e. more than 90 mg/l of sludgeformed.

It is clear that the experiment reported above and the results that itprovides go counter to the experience of a person skilled in the art andthe teaching that he can extract from the prior art. This is because:

-   -   the increase in flux through a membrane, by incomplete        clarification (with a coagulant dose markedly less than the        optimum clarification dose), and    -   the use of coagulating agents (especially ferric salt) in small        amounts, but nevertheless very much greater than the contents        normally prohibited by most membrane manufacturers and        suppliers, are completely contradictory to the commonly accepted        routine use of membranes.

Example 2: Treatment of Surface Raw Water

The trial was carried out on raw water from the Seine, thecharacteristics of which were the following:

Turbidity 15 NTU Organic matter 5 mg/l O₂ (KMnO₄ oxidizability) TOC 3mg/l UV 8 mg/l.

In laboratory testing, the dose which made the ZP equal to zero was 55mg/l of FeCl₃.

The output of treated water was 150 l/h. The steady flux through thecapillary ultrafiltration membrane was 80 l.h⁻¹.m⁻². When a smallquantity of coagulating reactant was used, by in-line injection of 2mg/l of FeCl₃ (expressed as pure FeCl₃), the maximum steady fluximproved by 30%.

The backwashing was carried out with 5 mg/l of chlorine for 30 secondsevery 30 minutes.

In this example, in which a surface raw water was pretreated with theobjective of treating it downstream by reverse osmosis, a major savingin membrane area was also found. All the TOC was not eliminated, but theflux was improved.

It follows from reading the description given above that the inventiondoes actually limit clogging of the membranes, by considerably improvingthe filtration capacity of the latter. This results in very substantialeconomic advantages, especially a reduction in the area of membrane tobe installed in order to treat the same volume of water.

Of course, it goes without saying that this invention is not limited tothe examples of implementation and/or application mentioned and/ordescribed here, rather it encompasses all variants thereof.

1. A method for maintaining the hydraulic performance of a micro orultra filtration membrane during filtration of effluent water containingsuspended matter by reducing membrane clogging and improving filtrationcapacity of the membrane, the method comprising the steps: providing aneffluent having as a principal pollutant colloidal organic suspendedmatters such effluent selectively being, municipal waste water, after abiological treatment and a separation allowing less than 20 mg/l ofsuspended matter (SM) to be obtained, water not requiring the priorremoval of organic matter and having a total organic carbon (TOC)content of less than 2 mg/L, or raw surface water, having a low TOC, ahigh colloidal concentration and containing less than 200 mg/l of SM;determining the dose (X) of a preselected coagulation reactant thatwould render the Zeta Potential of the effluent equal to 0; adding adose of a preselected coagulation reactant to the effluent directlybefore the effluent passes over the membrane, the added dose beingX/30–X/80 and by far insufficient to neither cause completeclarification nor neutralization of Zeta Potential; and directly passingthe effluent over the membrane after the dose has been added.
 2. Amethod for maintaining the hydraulic performance of a micro or ultrafiltration membrane during filtration of effluent water containingsuspended matter by reducing membrane clogging and improving filtrationcapacity of the membrane, the method comprising the steps: providing aneffluent having as a principal pollutant colloidal organic suspendedmatter, such effluent selectively being, municipal waste water, after abiological treatment and a separation allowing less than 20 mg/l of SMto be obtained, water not requiring the prior removal of organic matterand having a total organic carbon (TOC) content of less than 2 mq/L , orraw surface water, having a low TOC, a high colloidal concentration andcontaining less than 200 mg/l of SM; determining the dose (X) of apreselected coagulation reactant that would render the Zeta Potential ofthe effluent equal to zero; adding a dose of the preselected coagulationreactant to the effluent directly before the effluent passes over themembrane, the added dose being X/40–X/60 and by far insufficient toneither cause complete clarification nor neutralization of ZetaPotential; and directly passing the effluent over the membrane after thedose has been added.